1. Field of the Invention
This invention relates generally to a control circuit for beds having movable portions actuated by an electric motor, and more specifically, this invention relates to a control circuit for hospital beds in which electric motors are energized by full wave AC power signals to adjust movable portions of the hospital bed.
2. Description of the Prior Art
In some types of beds, especially those used by people who are bedridden for periods of time, such as hospital beds, it is desirable to be able to adjustably position the heighth of the bed. In addition, it is desirable to be able to adjustably position the angle of the patient's upper torso, and to adjustably position the knee support of the bed. Various type of adjustable bed arrangements have been utilized in the past, and more recently electric motors have been utilized to provide the driving force for positioning the bed portions.
One of the big advantages of electric motor drive is that it permits control by the patient of the positioning of the various bed portions. Thus, the patient can adjust the bed for his own comfort without having to call a nurse or exert undesired activity in mechanically adjusting the bed portions.
However, is such patient control is provided by a simple switch in the motor circuit, under some circumstances the patient could be exposed to dangerous electrical current levels, especially as the patient would normally be in a somewhat weakened state. Therefore, it is necessary to limit the exposure of the patient to electrical currents, both during normal operation and in the event of a fault or failure. One way to accomplish this is to utilize a separate low power source for the patient selecting circuit. The provision of such a supplemental power source creates many difficulties of its own. For example, if a battery is utilized, it means that the battery must be replaced at periodic intervals, with the attendant cost and maintenance problems. Accordingly, it is much more desirable to be able to utilize the available 120 volt AC power, which is also used to drive the motors, for the selecting function.
While some types of such systems have been developed in the past, some quite successful, they have generally been relatively complicated in order to obtain the desired isolation between the selecting and power portions of the system, and hence relatively expensive and more prone to failure.